docs: restore v2.5 docs snapshot and add missing v3.0 versioned docs (#808)

The v3.0.0 docs versioning run snapshotted docs using the pre-bump
version (`2.5.1`), which overwrote `version-2.5` with v3 content and
never created `version-3.0`. This PR restores the correct historical
`2.5` snapshot and adds a proper `3.0` snapshot from current docs.

- **Restore `2.5` to the last known good snapshot**
- Replaced `website/versioned_docs/version-2.5/` with the tree from
`5f7f7b1`.
- Restored `website/versioned_sidebars/version-2.5-sidebars.json` to
match that snapshot.
- Removes v3-only docs from `2.5` (e.g. `upgrading_to_v3.mdx`, custom
HTTP client/timeouts/typed-models concept pages).

- **Create a proper `3.0` versioned snapshot**
- Added `website/versioned_docs/version-3.0/` from current `docs/` + API
versioned output.
- Added `website/versioned_sidebars/version-3.0-sidebars.json` and
reformatted to 4-space indentation to match existing versioned sidebar
files.
- Ensures v3-only docs are present under `3.0` (including
`upgrading_to_v3.mdx` and new concepts docs), with the larger v3
`api-typedoc.json`.

- **Correct version registry ordering**
  - Updated `website/versions.json` to:
    ```json
    ["3.0", "2.5", "1.12", "0.6"]
    ```

- **Keep versioned snapshot clean**
- Removed root-gitignored carryovers from `version-3.0` (`.ruff_cache`,
`changelog.md`, `pyproject.toml`).